A method, apparatus, system, and computer program product provide the ability to modify a spline (e.g., a civil engineering spline). The spline, defined by a set of connected points, is obtained/acquired. A design constraint set is determined and may include an interpolation constraint (specifying a fixed elevation for a connected point), a slope constraint (specifying a bound on a slope between two of the connected points), and a curvature constraint (specifying; a maximum slope difference of a first slope and a second slope between three connected points). The spline is projected onto the design constraint set thereby modifying the spline by changing elevations of the connected points. The modified spline is then projected onto the design constraint set iteratively until the spline satisfies all constraints in the design constraint set.